In the AD&D universe, "Law" is a cosmic principle,
and in Mechanus it is predominant, unmixed with Good or Evil.
(Gary Gygax's
choice of "Nirvana" as
a name for this plane was perhaps not so fortunate as his others.)

Mechanus is a dimension where the living and the dead
dwell together, focused on Law. All creatures hear all
speech and see all writing in
their own
language, and can survive in any environment.
The whole is constructed of enormous interlocking gears,
each the seat of a fantastic civilization.
Don't expect to find unselfish love here. But the ordinary loves --
family, friendship, romance -- may be much as in our own world, if
more regulated and predictable.

The locals, especially the dead, are literal-minded and
often humorless. Faith communities tend to be bureaucratic
and legalistic, and more concerned with strict standards of behavior
than in spontaneous outpourings of kindness. Literary
models for Mechanus include the anthill in The
Once and Future King ("Everything Not Forbidden Is Compulsory").

In our world, we know two kinds of Law.

Natural Law is how the universe seems to work.
Surprisingly or not, scientists seem to be able to discover
fundamental rules that nature always obeys. This knowledge
enables to us predict, within its limits, the results of our
actions.

Human Law is invented and agreed-on
by people so that we can have
reasonable security to do what we want. Some thinkers
have said that God teaches us the basis of this kind of Law.
Others (including Sophocles and some modern philosophers)
believe that human law is discovered just as the laws of nature.
Still others believe that human law, and all other forms of culture,
are entirely human-made.

I am not aware of any reason to think that natural laws are
either benevolent or malevolent toward us, and the image of a
clockwork universe is familiar from classic philosophy.
Science has the powerful indirect effect of improving
human health and opportunity. Science makes it much easier to
make good decisions, and generally gives its
practitioners a social conscience.
But the study of nature is morally
neutral, and the business of science itself is both political
and tedious.

Human-made law is by itself can serve many different purposes.
Experience shows
that genuine democracy results in the greatest human good.
But our statutes range from laws founded on genuine love and strong
concern for others (Mount Celestia), the prescriptions of ideologues and
social engineers (Arcadia), the malices of identity groups
(Acheron),
the cruelty of status-seeking, conquest-hungry egomaniacs
(Baator),
or simply the social contract (Mechanus, "Let's all just figure out how to
get along.")

The theme of the social contract could be developed
as the basis for Mechanus, with each gear a different solution
found by a different kind of community. All these
worlds work and prosper, filled with
people who are content to
obey the law, raise families, and pursue their
interests without altruism and without malice.

From time to time, people forget why they
have made laws, and keep them for their own sake, out of habit
or for minority group identity.
Of course, silly statutes ("death for wearing a pinkie ring", as recounted
by one Planescape author) or arbitrarily-unrealistic statutes ("a woman's testimony
is given only half the weight of a man's", as codified in some "religious"
nations of our world)
are not Law, but the stuff of Chaos.

In our world, some movements have a lot of rules.
You have probably seen instances in which
this takes the place of common
sense and common kindness. You have many
examples for lawful-neutral clerics in your own community.
(There are plenty of chaotic-neutral
and true-neutral
clerics, too.)

Because this is a lawful plane, there may be some very large civilizations.
The "Fraternity of Order"
philosophic sect, which seeks control by understanding natural
law, has its headquarters on Mechanus at their Fortress of
Disciplined Enlightenment. ("This is happening for a valid
reason.") They have been wrongly described as arbitrary
legalists, but this cannot be true. They maintain a huge law
library. Brynn is a wheel millions of miles across and thousands
of miles thick, built of massive earthworks, where there is no
decay or corruption. The forests, mountains, seas, and plains of
Brynn are filled with growing plants, and when leaves and fruits
fall, they gradually turn to stone and become part of the land.
The same happens to anything else that is left here. The caverns
of Brynn are filled with beautiful crystals, while the skies bear
an endless procession of suns and moons. At the edge of Brynn,
one looks into literally nothing. The One True Church has its
headquarters on one of these wheels. ("We have all the answers.
Thou shalt not ask the hard questions.") So does the Temple of
Equanimity. ("Do it our way.") A few sects focused on obedience
and feudal duty are headquartered here. Myconid heaven is
somewhere here, inside a hollowed-out gear. It's all myconids
and fungi, with a central lake that seems infinitely deep and a
central palace only reachable if the Fungus King
wishes. There is no possibility of violence in the fungus realm, and
when a creature forms a violent intent, it is teleported away.
Somewhere is the headquarters of the "Multiversal Trading
Company", but no one has found it. Like the modrons, the
company deals in most goods, but never in holy or unholy things.
Customers on all planes make exchanges to and from headquarters
using teleport boxes. Haven is a well-run town inside a walled
cube, where the laws are reasonable and no one is oppressed.
Delon-Estin-Oti is a town where everybody seems to understand the
same things, and where communication is rare. There is nothing
here to interest visitors except for the possibility of getting a
prediction from one of the locals, who are very good at
interpreting patterns and processing data. Nemausus fell from
Arcadia into Mechanus, thanks to the Harmonium forgetting to love the sinner
despite hating the sin. It is a highly-ordered, bucolic world on the back of
a huge gear. Currently there is a battle between the modrons,
who would like to keep it in Mechanus, and some good souls who
would like to return it to Arcadia. Here you can find modron
towers, the remains of the Harmonium training camps, and some
chaotics who were freed from those camps when the plane
shifted. The most popular motto in modron territory is "Don't
question, but obey." By contrast, in areas of flesh and blood, popular
expressions are "This is happening for a valid reason", or
"Obedience is Freedom", or "Why be different?" Primitives will
discover a great stone city, governed by stern but just laws.
Paths are the Labyrinthine Portal, a series of teleportals from
gear to gear. The more lawful a user's mind, the greater the
chance of accurately selecting the destination. In fact, it's
said that anything you can imagine that makes sense
can be found on one of these
gears, and that the Labyrinthine Portal can take you there.
Public portals between the disks and to remote planes resemble
customs-houses between prime-plane nations. Entry from Automata,
on the Outlands, is by means of a gate with elaborate settings.
There's lots of red tape to get to use it, but it's accurate and
safe if used properly. Diamond-clear pools appear at designated
sites, and cannot be moved. Portals to the Outlands, Arcadia or Acheron may be
arches of red tape. Other attractions on various disks include a
region where ability points can be bargained up or traded, a
library where most knowledge is contained (there's lots of red
tape to get user's privileges, and don't expect to learn much
about virtue or vice here), a garden where grow the peaches of
longevity (restore 1 year of youth each without any risk -- but
hard to get), a world of ferocious hawks and hawk-people, a world
of robots, and many anthill and beehive worlds.

Spell alterations in
Mechanus: Only lawful creatures can be summoned, but they
will be obedient. They will twist the instructions of chaotic
summoners. If the alignment is identical, the creature will do
its best to serve well. Chaotics cannot divine. Illusions do
not even manifest. Necromancy requires 1 hp for a first-level
spell, and 4*(level-1) for others. Wild magic fails. Elemental
magic requires a bit of the element native to the plane, and
there is good earth and water only in Nemausus. Spell keys
might be able to allow a chaotic to use divinations, but keys to
overcome other limitations of the plane seem unlikely.

Wizardly spell keys are
one-twelfth of a cog, colored appropriately and
marked with the sigil of a spell school. If twelve such keys,
one for each school, are joined and absorbed by a wizard, they
will never be lost, but all spells cast off the plane will be at
half effect. Or, for necromancy, a bit of a modron's body and a
prayer in Chinese. Power keys for clergy are little gears, and
they return to sect headquarters, along with the user, at the end
of the day. Lawful owners will always get them back, perhaps with
the key renewed. Woe to anyone who has stolen a power key!

The suggested color for pools from the astral is diamond.
Ethereal curtains might be white.

The dead are immune to fire and cold.

The plane is "strongly law-aligned". Chaotic creatures have -2 on all intelligence, wisdom, and charisma checks.

I respectfully suggest that the actual effect might be

-1 on all intelligence, wisdom, and charisma checks for all non-lawful, non-chaotic creatures
-2 on all intelligence, wisdom, and charisma checks for all chaotic creatures
Good-based spells (non-chaotic) are unaffected.
Evil-based spells (non-chaotic) are unaffected.
Law-based spells work as if caster were 4 levels higher.
Chaos-based spells simply fail.

The Fourth Edition has brought forth only a few clockwork creatures.
Erathis is "unaligned" but clearly lawful-neutral, and those who
still hold the ideals of this not-fun-to-play alignment
might someday have their own astral realm.
Perhaps visitors would get bonuses or penalties
to intelligence, wisdom, and charisma-based skill checks
depending on how much their behavior has been in keeping with the ideals
of the locals.

In keeping with the flexibility of the third and fourth editions and the
backgrounds of many players, perhaps Mechanus is essentially a world
where like-minded spirits meet. It looks and works like our own world,
except that the laws aim at stability without either beneficience
or malice, and where all social contracts are perfectly
clear.
NPC attitudes are typically "indifferent"; the locals respond best
to evidence that visitors want to be law-abiding community members.
The modrons of Mechanus are a spiritual, non-human (or formerly human) race devoted to promoting the community's
ideals
among the living by encouragement and subtlety, rather than by force.
The dead find communities matching their own ideals and interests,
and continue to live much as they did on earth, though no longer able to visit the
Prime Plane.

Instead of the "gods" of polytheism, each living Lawful Neutral
divine spellcaster is
sponsored (and monitored)
by a prayer fellowship with similar interests based on Mechanus.
For the fourth edition, I suggest no penalties for divine spellcasters from elsewhere. For earlier editions, I respectfully suggest that the only penalty for such a cleric
on a differently-aligned outer plane is the loss of one spell
of the highest available level for each plane removed, with the Outlands two planes from Mechanus, Elysium, Limbo, and the Gray Waste. When one level
is depleted, spells of the next highest level are lost. Thus a cleric
sponsored from Mechanus would lose one spell on Arcadia or Acheron,
four spells on Limbo, and six spells on the Abyss or Arborea. Moving to the Outlands loses two spells.

A world of cold reason and perfectly-articulated social contracts
would be as fascinating and thought-provoking
as any rules-intensive world ever visited by adventurers.

Referees might not want players to realize that they have
entered Mechanus. Depending on the site of arrival,
visitors might simply recognize a mostly-benign police state
with an impressive culture.

Look for an entry to Mechanus when...

mathematicians and scientists lose themselves
in their work;

a culture like ancient Sparta adopts severe
laws that render them successful and secure;

a minority group develops or maintains a complex
code of behavior that has no direct impact on safety
or happiness, merely to keep itself together;

a culture approves of its martinets;

political philosophers talk a great deal about "ensuring
stability to promote prosperity"

businesses (legal, illegal) have strong corporate
traditions and rituals but no social conscience;

a culture prescribes behavior based on dyadic
relationships and obedience rather
than love and kindness ("You should let your
parents choose your job and your
marriage partner without even expressing
your preferences."
"You have no reason to donate blood that would
go to a stranger.")

groups of bright college men are united by a
common narrow focus on math, music, chess, and ultraconservative
religion. You've met the type.

people find peace by letting others make their decisions for them. (Life has taught
me that this promises trouble for you and those around you.)

The Mahabharata,
the Indian national epic, is great browsing. There are obviously multiple authors,
but the major theme is dharma, obedience to law as a guiding principle
of life. The central story is a war between the Pandavas, who follow dharma,
and their enemies who do not. The human characters discover they are fighting
a cosmic struggle for Law against Chaos, etc., etc. The Pandavas are light-skinned,
as were the victorious Aryan invaders, whose
Indo-European culture was probably successful
because they were the original tamers of horses, and because their military leaders
(in India, the Kshatriya caste) accepted the authority of non-combatant civilians
(in India, the Brahmin caste). Most of the poem mirrors a warrior's ethic.
Do as is expected of one in your station in life. Do not be soft-hearted.
Act firmly and resolutely. Take revenge.
Keep your word even when it makes no sense
and without regard to the consequences. You should be fair and upright
with those who follow dharma. Whether you may lie and cheat in order to
defeat those who do not follow dharma is a subject of disagreement among the different strata.
Things are predestined. What goes around comes around.
There is an overriding emphasis on Law.
The harshness of the times sometimes shows, but a few of the strata show a tendency
to kindness as well.

On Mechanus itself, look for:

dull, drab factories and army camps;

anthill and beehive worlds;

clockwork creatures and robot worlds;

communities with slaves who are
not treated with arbitrary cruelty;

egalitarian societies where there is opportunity
without altruism;

the Multiversal Trading Company, dealing in
everything except those items that promote good,
evil, or chaos.

"Li Po" likes science, math, libraries, programming, medicine,
and helping people obtain justice in court. In the process,
he hopes he never forgets the "why" -- which is the good of others.
To forget love is to be forever lost among the gears.

How can you role-play a non-good cleric?

In college, I knew a medical student from a strong _____
background. He had even fought for a time in the _____ army.

One day, he was crying. He told me that his family had disowned him.
Why? He had been dating a _____, a non-_____ girl.

His family sent out announcements of his death, and held a funeral
for him.

This disturbed me deeply at the time, and it still does over thirty
years later. Sadly, I've run into similar things from people pretending to follow
each of the great world faiths. It seems a terrible perversion of
what should be a good thing.

No religion has a monopoly on good and bad, and most _____ people
are not like this. But I'm glad I haven't been asked -- or asked anybody --
to choose minority group
identity over family love or the common bonds that should unite the human race.

Even if you do not profess lawful alignment, please respect
TSR's copyrights. Even if it weren't the law -- we'd still
do it out of gratitude.